Generation and Reversible Cyclisation of Furfurylic Radicals

The chemistry of free radical intermediates has undergone a renaissance with the recent advent of alternatives to toxic, stoichiometric reagent systems and thermal reaction conditions. In this context, we describe an attempt to effect an isomerisation reaction comprising overall 5-exo-trig radical cyclisation preceded and followed by electron and proton transfer steps. In a follow-up study the 5-exo-trig cyclisation of furfurylic radicals, generated unambiguously by classical methods, is here shown to be synthetically viable, reversible, and equally effective with either an electron-deficient or an electron-rich alkene component. The experimental results are compared with DFT transition state and ground state energy calculations. [GRAPHICS]